Three generations of selective breeding improved Rainbow trout (Oncorhynchus mykiss) disease resistance against natural challenge with Flavobacterium psychrophilum during early life-stage rearing

Authors: Wiens, Gregory - Greg; Palti, Yniv; Leeds, Timothy - Tim

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/31/2018
Publication Date: 8/1/2018
Citation: Wiens, G.D., Palti, Y., Leeds, T.D. 2018. Three generations of selective breeding improved Rainbow trout (Oncorhynchus mykiss) disease resistance against natural challenge with Flavobacterium psychrophilum during early life-stage rearing [abstract]. Aquaculture. 497(2018):414-421.

Interpretive Summary: Selective breeding for disease resistance offers new opportunities for improving fish welfare in aquaculture. At our location, we have used selective breeding to develop three lines of rainbow trout (resistant, control, and susceptible lines) that differ in their resistance to Flavobacterium psychrophilum, the cause of bacterial cold water disease (BCWD). In this study, we evaluated the lines at a hatchery site that routinely experiences BCWD outbreaks to confirm that resistance as assessed in the laboratory is similar to resistance on the farm when fish are naturally exposed to the pathogen. In addition, we performed laboratory challenges to experimentally confirm susceptibility. Eggs from all three lines were combined into a single egg pool and sent to a hatchery site, and reared adjacent to an age-matched hatchery line, Fish Lake-DeSmet (FLDS). Survival was followed for 80 days after fish started feeding, a time period when outbreaks frequently occur. An outbreak of BCWD was diagnosed in the FLDS group that first started on trial day 42 and peaked at day 52. Analysis of fish from the adjacently-reared pooled lines that died between days 42 and 69 of the trial identified that most fish were from the susceptible and control lines, with only a few from the resistant line. Furthermore, sampling of live fish on day 78 failed to detect the pathogen in the resistant line while pathogen was detected in the control- and susceptible-line fish. We also experimentally challenged the four genetic lines with two isolates of F. psychrophilum. The resistant line fish exhibited significantly higher survival than either the susceptible or FLDS lines. The resistant exhibited higher survival than the control line following challenge with a standard challenge isolate but not a field isolate. These data suggest that three generations of selective breeding resulted in reduced mortality and pathogen prevalence on the farm and that the impact of breeding was primarily through the reduction of highly-susceptible animals.

Technical Abstract: Abstract: Bacterial cold water disease (BCWD) is a frequent cause of mortality in freshwater rainbow trout (Oncorhynchus mykiss) and the implementation of a family-based genetic selection program to improve disease resistance has been previously described. To quantify and understand the consequences of selection, three lines of outbred rainbow trout were created by divergent selection: ARS-Fp-R (resistant), ARS-Fp-C (control) and ARS-Fp-S (susceptible). Herein, results from both field and laboratory studies are described comparing survival of third-generation ARS-Fp-R line eggs/fish stocked at equal proportions to control and susceptible lines and reared together in a common environment. The survival of the pooled-line cohort was compared to the survival of an age-matched hatchery line, Fish Lake-DeSmet (FLDS) during early life-stage rearing. A typical BCWD outbreak occurred in the FLDS line while a more limited mortality event occurred in tanks containing the pooled lines. Mortalities from days 42-70 of the trial were saved for pedigree assignment and pathogen load measurement. After accounting for differences in early post-hatch survival, ARS-Fp-R line fish demonstrated two-fold lower BCWD-related mortality while ARS-Fp-S line fish demonstrated two-fold higher mortality; however, there was not a survival advantage of the ARS-Fp-R line fish over ARS-Fp-C line fish. At the end of the trial period, F. psychrophilum was not detected in spleen tissue of live-sampled ARS-Fp-R line fish while 10.3% of the ARS-Fp-C line and 36.4% of the ARS-Fp-S line fish exhibited detectable loads. Experimental challenge of the genetic lines with two different F. psychrophilum strains confirmed significantly higher survival of the ARS-Fp-R line fish as compared to either the ARS-Fp-S or FLDS genetic lines, albeit the susceptibility of the FLDS line was confounded by slower growth rate. These data demonstrate that three generations of artificial selection translated into improved survival following either natural or experimental exposure to F. psychrophilum. These data also suggest that hatchery lines, such as Fish Lake-DeSmet, may have higher relative susceptibility to BCWD thereby contributing to location outbreak severity.